An inkjet head for use in an inkjet printer includes a plurality of pressure chambers for accommodating ink, a nozzle plate which is provided on one end of the pressure chambers. A plurality nozzles are provided in the nozzle plate for ejecting ink droplets to the pressure chambers respectively. A plurality of piezoelectric actuators are provided correspondingly to the pressure chambers to give vibration to the pressure chambers respectively through a vibration plate.
In this kind of the inkjet head, when the piezoelectric actuator is driven, vibration is given to the pressure chamber corresponding to the actuator. This pressure vibration changes the volume inside the pressure chamber and an ink droplet is ejected from the nozzle corresponding to the pressure chamber. The ink droplet is applied to a recording medium, such as recording paper and forms a dot on the recording medium. By forming these dots continuously, the inkjet head forms a letter and an image on the recording medium according to image data.
In the inkjet head, it is preferable that ink droplets are ejected stably from the viewpoint of printing accuracy. There is known a first example of a driving waveform in which, with a predetermined cycle (AL) as a unit, voltages −V, 0, and +V are applied to three AL periods.
This method may eject ink droplets stably. This method, however, does not obtain a sufficient ejection amount because a difference in the adjacent voltages is V. A second example of a driving waveform is known in which −V is applied in the first AL period, +V is applied in the next second AL period, and 0 is applied in the next AL period. In this case, a difference between the first AL period and the next second AL period is 2V, and sufficient ink may be ejected.
However, when +V is applied at the second AL period, ink ejection gets unstable.